I doubt Arriva will have planned what they intend to provide as yet.
However, the last time 26 December was a Saturday was 2020. Arriva ran a special timetable on routes 2, 101, 166 and 700. Sunday services applied on 27th, 28th and 1st; Saturday services on 29th-31st. There were early finishes on both Christmas Eve and New Year's Eve.
If I recall correctly, KCC contracted route 151 runs a Sunday service on Boxing Day - I'm sure it was the only bus running in mid Kent last year.
I've no connection with Arriva, Kent County Council or Medway Council so can't give any sort of official answer, but I'd say something along those lines is the most likely.
But don't expect the definite information to be out until some time in December - legally, operators only have to give the Traffic Commissioners / local authorities 3 weeks' notice of special arrangements for weeks involving bank holidays - although most have been persuaded to do it earlier so that Traveline and all the other websites / apps have the information in plenty of time. I'm still usually chasing operators for information (in my local authority role) in to the first week of December.
Most bus operators (and their staff) would see 26 December as Boxing Day, whatever day of the week it falls on, and their locally agreed special conditions would apply to 26 December, not the bank holiday in lieu. (It varies by operator whether those arrangements are no services, limited services and premium pay rates or whatever. TFL is probably an outlier in having a full Sunday service, although some London operators apply special duty conditions, and there's complications round the absence of night buses / 24 hour route 'night journeys' at start of service.)
And fairly sure the railways largely close on 25 and 26 December whatever days of the week they fall on.
Broadly, the above is what happened at bus operator I worked for in 2020 - we ran our version of limited / special service on Saturday 26 December, then Sunday service Sunday 27th and bank holiday Monday 28th. And (it's longer ago) but think we did similar (at a different bus operator) when Christmas Day was a Saturday and 26 December was a Sunday followed by two bank holidays, where Sunday 26th was special / limited service, not standard Sunday service.
In 2020, I seem to remember that there was a discussion with one of the local transport authorities, who tried to argue that we should run a full Saturday service on tendered routes on 26 December as it was a Saturday not a bank holiday. I wasn't personally involved in telling them not to be so silly (I think it would have involved running an evening service on a route or two that wasn't going to operate in the daytime) but we ended up running our planned special schedule.
And an increasing number of operators are stretching the 'Saturday service' days to more than just the (non bank holiday) weekdays between Christmas and New Year - it would not be a big surprise if some operators do Saturday service from Monday 21 to Thursday 24 December (with early finish on 24th) this year.
